This strap idea is merely a bandaid for a problem that should be resolved in a different manner. The swelling battery basically will disconnect from its corresponding contacts and provide the pilot with an earthbound chunk of plastic along with some other material.
With the numerous(beyond the twenty mark)M2 series aircraft I’ve run through, I have never contemplated attaching anything other than a strobe light. If I ran across a battery that had anything other than a flat bottom, it was tossed.
A wallet should not be a consideration when safety is concerned. If reputable batteries aren’t available, there are other DJI products out there with available batteries.
Be safe please.